Brush seals have been proposed to minimize leakage through the gap between stationary and rotating components, particularly where the leakage is from a higher pressure area to a lower pressure area. For example, in gas or steam turbines, brush seals have been used to minimize the leakage of air or steam, respectively, between the rotary and stationary components and specifically to seal the stationary component with the rotating shaft of the turbine. Such brush seals conventionally have wire or ceramic bristles, often welded or otherwise affixed to a backing plate. The free ends or tips of the bristles lie in contact with the rotational component.
Bucket covers in turbomachinery, for example, steam turbines, are typically integral with the buckets and are machined while on the rotor. This results in excellent radial alignment of the bucket covers until the rotor spins at operating speed or its rated overspeed. At such speeds, buckets in high-pressure sections of the turbine shift, which results in a radial misalignment of the covers, for example, in a range of 0.003 to 0.005 inches. The notch bucket cover on tangential entry dovetail buckets can be misaligned radially by 0.008 inches or more. Deformation during rotation can increase the misalignment of the notch bucket by up to 50%, the rotation not significantly affecting the alignment of the remaining buckets. Corresponding misalignments in an intermediate pressure section of a steam turbine are anticipated to be significantly larger, particularly in later stages.
It has been demonstrated that running surfaces which have steps such as those expected between adjacent covers of integral cover buckets can lead to accelerated wear of the bristle tips as those tips pass the covers. Such wear, for example, during a transient radial excursion of the rotor can lead to excessive clearance and performance degradation. Severe damage to the brush seal can also occur and wear on the bristle tips can be excessive. One effort to address the problem of damaged or worn brush seals has been to provide bristles of differential abrasive qualities. For example, bristles of higher abrasive qualities may initially smooth out the rough surfaces of the shrouded free ends of the buckets, while the less abrasive bristles conform to the shape of the now smooth surfaces to achieve good sealing characteristics. The present invention, however, provides a different approach to the problem of avoiding damage and excessive wear to the brush seals in the environment of a seal between the bucket covers of rotating turbine blades and a surrounding sealing segment or shroud by altering the configuration of the sealing surfaces of the bucket covers.